Peel onions and garlic. Chop onions finely and chop garlic finely. Heat oil in a pot. Sauté onions and garlic in it. Add frozen spinach and about 2 tbsp. water. Cover and allow to defrost at medium heat for 16-18 minutes, stirring from time to time.
In the meantime let the filo pastry rest for 10 minutes at room temperature. Wash parsley, shake dry and chop coarsely. Crumble the feta. Melt butter.
Drain the spinach and squeeze a little. Mix with parsley, feta, breadcrumbs, milk and whole eggs. Season to taste with salt and pepper.
Grease a springform pan (approx. 26 cm Ø) and dust with flour. Roll out the dough and place 2 pastry sheets in the form so that the sheets hang over the edge of the form. Brush the dough with butter.
Place 4 more leaves on top and spread with butter.
Preheat oven (electric cooker: 175°C/circulating air: 150°C/gas: level 2). Pour the spinach mixture into the mould, place the excess dough over the spinach. Place the rest of the dough sheets on top, brushing each with butter.
Fold the protruding dough towards the middle and press it down. Bake in a hot oven for 25-30 minutes. Whisk the egg yolk and 1 teaspoon of water. Spread the top of the dough after about 20 minutes, sprinkle with sesame seeds. Eat hot or cold.